Known in the art are several industrial applications, of electrolytic or electrometallurgical type, which use electrodes deputed to the evolution of a gaseous product, whose production constitutes in some cases the main object of the process (such as chlorine evolved at the anode in the electrolysis of alkali chloride or hydrochloric acid). In other cases, the evolved gas is a simple reaction by-product (such as hydrogen evolved at the cathode in the alkali chloride electrolysis, or oxygen evolved at the anode in the cathodic metal electroplating, typically in the galvanic industry). In both cases, one of the main goals in the production of gas evolving electrodes is the high electrochemical activity, which must permit to operate with overvoltages as low as possible in order to increase the overall energy efficiency of the process. It is therefore customary to carry out such reaction on catalysed surfaces, also when the gas evolved at the electrode is a mere by-product. As the materials with the best electrocatalytic properties are very expensive, this category fundamentally consisting of the platinum group metals and their oxides, their use is limited to thin surface layers deposited on a conductive matrix. In particular, it is well known to the experts in the art the use of metal substrates combining good conducting characteristics and resistance to corrosion, having at least one surface coated with a thin layer of noble metals and/or oxides or alloys thereof; embodiments of this kind are described for example in patents such as U.S. Pat. No. 3,428,544, U.S. Pat. No. 3,711,385, and many others. Resistance to corrosion is a very critical parameter especially in the case of electrodes suitable for operating as anodes, where the aggressive action of the electrolytes is enhanced by the electrochemical potential. For this reason, the anodes for industrial electrolytic and electrometallurgical applications are made preferably starting from valve metal substrates, that is metals resisting to corrosion in view of the protection imparted by a thin superficial layer of inert oxide. Among these valve metals, the most commonly used is titanium, due to considerations of cost and workability. The electrochemical characteristics of titanium substrates coated with catalysts based on noble metal oxides are commonly considered as more than satisfactory as anodes for gas evolution in nearly all industrial applications. Conversely, their lifetime, especially in the most critical operating conditions (very aggressive electrolytes, very high current density, etc.) constitutes, in many cases, a problem still not completely solved, although an exhaustive technical literature witnesses the fundamental progresses made in this field. A long electrode lifetime is the essential condition for the industrial success in electrochemical applications, not only because in case of deactivation a new electrocatalytic coating must be applied, which is expensive on one hand in terms of material and manufacture, and on the other hand in view of the production loss associated with the shut-down of the plant during electrode replacement. As the noble metals used in the formulation of the electrocatalytic coatings are per se immune from corrosion in the usual operating conditions, the main cause of deactivation consists in the local detachment of the coating from the substrate, with the consequent corrosion or passivation of the latter. This detachment is favoured by the same gas evolution, due to the mechanical action of the bubbles generated on the surface, and the phenomenon is further increased at high current density. In particular, in some electrometallurgical applications with anodic evolution of oxygen, for example in the zinc plating of sheets for use in the automobile industry or in the production of thin copper foils for the electronic industry, the anodic current density exceeds 15 kA/m2.
Another source of instability for the adhesion of the coating to the substrate may derive from the porosity of the former, allowing infiltration of the electrolyte in direct contact with the non protected metal substrate. In such cases, if localised detachment occurs even in microscopic areas, the substrate may passivate, forming an often scarcely conductive oxide, which grows between the substrate and the electrocatalytic layer without any major physical detachment of the latter taking place.
In order to obtain a sufficient anchoring of the catalytic coating, it has been found advantageous since the advent of these types of electrodes to provide a certain superficial roughening of the substrate, for example by means of a sandblasting treatment, or by controlled etching with a corrosive agent. The surface roughness favours the reciprocal intermixing of the substrate and the catalyst, the latter being obtained by thermal treatment of a precursor compound in the form of a paint applied to the substrate. In the case of titanium, for example, well known are the abrasive treatment made with sand, sand mixed with water or corindone, and the etching with hydrochloric acid; these procedures permit to obtain electrodes suitable for use in some industrial applications, with the provision that the electrodes still need to be subjected to rather frequent periodical reactivation. In this regard, the electrometallurgical processes with anodic evolution of oxygen are to be mentioned among the most penalised applications, especially in the case of operation at a current density higher than 10 kA/m2. Also for low current density processes, however, as in the case of electrowinning of metals from acidic solutions deriving from the primary dissolution of minerals, problems arise albeit of a different nature; among these are those associated with the impurities always present in the electrolytic baths, some of which have an extremely deleterious effect on the passivation of titanium substrates. A typical example is given by fluoride ions, capable of complexing titanium, thus destroying the relevant protective film with the consequent corrosion of the underlying metal matrix, especially in the areas previously exhibiting micro-defects in the adhesion of the electrocatalytic coating to the substrate.
For this reason the use of interlayers with suitable characteristics for inhibiting corrosion, interposed between the metal substrate and the electrocatalytic coating, has been proposed in several cases under different forms, aimed at blocking the localised corrosive attack taking place where the inevitable micro-defects are present in correspondence of such barrier. An example of interlayer, based on ceramic oxides of valve metals, is described in European Patent EP 0 545 869, but other examples of interlayers are known in the art.
The use of an interlayer is often decisive to increase the lifetime of the gas evolution electrodes to a great extent. However, this is not free from inconveniences. The ceramic oxides, for example, have an electric conductivity lower than that of titanium and of the materials used for electrocatalytic coatings, thus introducing resistive penalties which negatively affect the overall electric efficiency of the process; furthermore, they may alter the surface morphologic characteristics, as they mask the underlying rough surface with a negative influence on the adherence of the catalytic coating. The control of their thickness and of the overall morphologic characteristics after their deposition is therefore mandatory. The definition of the optimum roughness parameters of electrodic substrates suitable for applying an electrocatalytic coating thereto is disclosed for example in EP 0 407 349, assigned to Eltech Systems Corporation, USA, where it is specified that, in order to ensure a high quality adhesion of the coating, an average surface roughness not lower than 250 microinches (about 6 micrometers), with an average frequency of at least 40 peaks per inch (basis a profilometer upper threshold limit of 400 microinches, that is about 10 micrometers, and a lower threshold limit of 300 microinches, that is about 8 micrometers) is needed. Such roughness is obtained by etching, in order not to damage the surface with abrasive treatments, such as sandblasting and the like.
This limitation is easily understandable when the two fundamental characteristics of the claimed surface profile are taken into account, namely the average roughness and the frequency, the latter being, in more common terms, the population of peaks observed on a reference length, for example 1 cm. The indicated average roughness may indeed be obtained without much difficulty also by sandblasting, provided that the abrasive powder has a suitable particle size. The resulting inconvenience is a strong hardening of the metal with the relevant internal stresses, which may be released by a subsequent annealing treatment at 500-700xc2x0 C. in the case of titanium. Obtaining the frequency, or population of peaks, is conversely rather problematic and requires a careful optimisation of the sandblasting parameters, such as the type of nozzle, the pressure of the air jet, the distance and the angling of the nozzle with respect to the metal surface, as well as prolonged treatment duration. This last factor in particular causes the nucleation of surface defects such as micro-cracks or bendings which constitute the starting points for the so-called xe2x80x9ccrevice corrosionxe2x80x9d during operation as the anode. Furthermore, checking that the desired frequency or population of peaks is actually achieved requires long times which increase the production cost and are intrinsically problematic, as the best results are obtained not only when the number of peaks per centimeter is the desired one, but also when the peaks are distributed in a more or less uniform way and not clustered together, thereby leaving some residual areas with insufficient roughness. In any case, although the teaching of EP 0 407 349 is important to provide the substrate with the best possible morphology, it still remains insufficient to impart an adequate lifetime to the electrodes, and especially to the anodes, for many critical industrial applications. For this reason, a subsequent European Patent (EP 0 576 402) by the same applicant demonstrates that the use of the roughness parameters of EP 0 407 349, combined with an interlayer based on ceramic oxides which in its turn retains the same surface parameters to allow the correct anchoring of the catalytic coating, prolongs the electrode lifetime to a surprising extent. However, even at first sight the applicability of such invention appears extremely problematic as, in addition to the cited problems associated with the difficulty of obtaining the roughness of EP 0 407 349 it is also required that the interlayer be, on one hand, sufficiently protective, and on the other hand, not so thick as to modify the surface roughness parameters. In this regard, claim 1 of EP 0 576 402 specifies that the method for producing the electrode requires a check of the surface roughness after the interlayer deposition and before the coating application. It is evident therefore that it is necessary to operate within very strict parameters, which are also difficult to control; moreover, checking the roughness after deposition of the interlayer grants the optimum morphology for adhesion of the subsequent electrocatalytic coating, but not the fact that the same interlayer is compact and coherent enough to perform its protective function effectively over the whole electrodic surface.
It has been therefore demonstrated how in the prior art there exists the need to identify a configuration of electrode based on a metal substrate provided with superficial morphological characteristics suitable for obtaining the best anchoring of an electrocatalytic coating thereto, at the same time preserving said substrate from passivation phenomena, even in critical operating conditions, without having to rely on overly expensive and complex surface pre-treatment procedures and without any interlayer between the substrate and the electrocatalytic coating.
It is an object of the present electrode to provide an electrode for gas evolution comprising a metal substrate having a controlled surface morphology and an electrocatalytic coating without any interlayer, capable of overcoming the limitations of the prior art.
Under another aspect it is an object of the present invention to provide a method for the preparation of an electrode for gas evolution which comprises a metal substrate having a controlled surface morphology and an electrocatalytic coating without any interlayer.
Under another aspect it is an object of the present invention to provide an electrolytic process making use of at least one electrode for gas evolution which comprises a metal substrate having a controlled surface morphology and an electrocatalytic coating without any interlayer, on whose surface a reaction of evolution of a gaseous product takes place.
Under another aspect it is an object of the present invention to provide an electrometallurgical process of cathodic metal plating making use of at least one anode which comprises a metal substrate having a controlled surface morphology and an electrocatalytic coating without any interlayer, on whose surface a reaction of evolution of a gaseous product, for example oxygen, takes place.